1. Field of the Invention
The present invention relates to a geartrain, and more particularly, to a geartrain for a five-speed automatic transmission used in vehicles, which can be designed to be compact in size and lightweight by reducing the number of frictional elements and planetary gearsets.
2. Description of the Related Art
Generally, automatic transmission systems for vehicles comprise a transmission control unit (TCU) which automatically controls shift ratios according to changes in a running condition of the vehicle.
The above-described TCU controls a plurality of friction elements provided in a geartrain to either operative or inoperative states to select one of the three essential elements of the planetary gearset (a sun gear, a ring gear, and a planet carrier) to be an input element, another a reaction element, and the last an output element, thereby controlling the number of output revolutions.
The geartrain is generally designed to realize four forward speeds and one reverse speed. However, with the development of high-power engines, geartrains have been developed to realize five forward speeds and one reverse speed.
The 5-speed geartrain is comprised of three simple planetary gearsets and at least six friction elements.
However, in such a geartrain for a 5-speed automatic transmission, to drive the vehicle in fourth or fifth speed-overdrive, there is a need for an element which rotates at a higher speed than that of a final output element. This results in power loss. In addition, the large number of friction elements results in a heavy and large-size automatic transmission.